|Hello again-like I said it COULD be swimmer's itch. I was just going by the info you posted. Your doctor will tell you after seeing your rash. There was no info about the exact location of the larvae other than it is IN the water, and is less concentrated in deeper water. They are microscopic so you can't see them anyhow.
I have heard of beaches being temporarily closed down due to large cases of swimmer's itch. Not everyone is susceptible to the parasites (meaning they don't have an allergic reaction--the rash). It would be interesting to find out if anyone else on your trip got a rash also--but as you see there are variables about who gets symptoms.
Whether or not you take a steroid is up to you if it is offered by your doctor. Read the info here or online to make an informed decision beforehand as many people experience side effects even from one shot or a few days of pills. From what I have read, the larvae die in a few days anyhow and I would imagine your symptoms will clear up after that on it's own if you have swimmer's itch. Good luck and keep us informed :)